Brightening the clouds touted as a way to save corals

CORALS are in dire straits, so it will take an ambitious plan to save them. Will cooling the climate locally by making clouds brighter help?

Corals face a perfect storm of threats including acidifying oceans, invasive predators and tropical storms. Warmer waters also put them under stress, making them expel photosynthetic microbes that provide most of their energy as well as their vibrant colours, leaving them a bleached white. Without symbiotic microbes, the corals will die.

To try to reduce this bleaching, Alan Gadian of the University of Leeds in the UK and colleagues simulated a form of geoengineering called marine cloud brightening. Ships would spray seawater into clouds to create extra water droplets, making the clouds whiter. This could reduce the ocean temperature locally because more sunlight would be reflected back into space.

When the model's carbon dioxide levels were double that of pre-industrial times – a level expected to be reached by around 2050 – bleaching events became much more common, compared with a control scenario with lower CO2 levels, where events were rare. But when three areas of ocean that tend to have lots of low-lying cloud were targeted with cloud brightening, levels of bleaching were the same as the control run (Atmospheric Science Letters, doi.org/m5n). The project could cost more than $40 million a year.

"This is an attempt to do something good," says Tim Lenton of the University of Exeter in the UK. "But I'm not sure it's a brilliant idea." If the programme stopped for any reason, the ocean would heat up quickly. "The worst possible thing for the corals is rapid warming."

Bleaching is only one threat, says Lenton. On the Great Barrier Reef, it has caused 10 per cent of losses: storms and starfish were responsible for far more.

This article appeared in print under the headline "Give every cloud a sea salt lining to save reefs"

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